This invention relates to the field of powders or cakes of M type hexaferrites, that is to say constituted of particles of magnetoplumbite of general formula AFe12O19 where typically A represents Ba, Sr, Ca, Pb, the element A and/or Fe being able to be partly substituted by other chemical elements.
These powders are used to manufacture ceramic magnets by compression with orientation of the particles under a magnetic field, and then sintering of oriented and compressed particles.
These powders can also be used in the production of magnetic recording media.
The usual method for manufacturing powders of ferrite particles is as follows:
the raw materials are provided in the form of powders, typically iron oxide Fe2O3 and strontium carbonate SrCO3 in the case where A represents Sr,
the powders are mixed with excess Sr relative to the formula of the ferrite, typically a molar ratio Fe2O3/SrO of about 5.5 instead of 6, a ratio which corresponds to the M phase formula (SrFe12O19), and possibly after incorporating additives,
the powder mixture is shaped, usually by granulation, or powder compression,
type M ferrite is formed by calcination of the powder granules in an oven at about 1200xc2x0 C., and after cooling, ferrite granules are obtained, typically with an apparent density da of at least 4, that is a porosity typically lower than 20% compared to that of an M ferrite of Sr of density dx equal to 5.11.
the calcinated granules are ground in several stages, with a first rough grinding followed by one or several fine grindings, comprising classification by particle size, the final ferrite powder obtained being exempt of any big aggregates constituted of primary particles (elementary particles), even fine ones.
It is to be noted that in this process, an excess of Sr is used, which leads to the formation of a second liquid phase, which facilitates the calcination reaction and makes the process less difficult to implement. But in this case, it is generally necessary to add silica to limit crystalline growth.
Afterwards, in the case of production of magnets, the fine powder obtained is shaped, to the form of the final magnet, after being dispersed, with orientation of the primary particles under magnetic field and compression.
The compressed product is then dried, sintered and if necessary machined to the required dimensions.
The final performances or final magnetic properties obtained, typically the remanence Br or coercive force HcJ, depend in particular directly on the morphology of the ferrite particles, the remanence Br depending in particular on the ability of the primary particles to be aligned in parallel, and the coercive field HcJ depending in particular on the size and shape of the particles.
Thus, the presence of aggregates formed of primary particles aligned randomly is greatly detrimental for obtaining high magnetic properties, the aggregates being polycrystalline and formed of particles oriented in a haphazard way.
The grinding phase of the state of the art method is thus an important stage of the method, both because of its length and because of its cost, including investments, and through its consequences concerning the final properties of the magnets.
The principal problems associated with this grinding are as follows:
on the one hand, as calcinated ferrites are difficult to grind, one observes significant wear of the grinding equipment and also ferrite pollution by outside elements from the grinding media, which have negative effects on the magnetic properties;
on the other hand, since the elementary particles are very difficult to separate, the final grinding stage is very longxe2x80x94and consequently there is a high proportion of smalls (particles of particle size lower than 0.3 xcexcm), with negative effects, both concerning the shaping of the ferrite powder for forming magnets, and concerning the possibility of aligning these low dimension particles under a magnetic field, or furthermore the possibility of reagglomeration of smalls, the high possibility of recrystallisation during sintering etc . . .
The consequences are a lowering of the final magnetic properties, the values of Br and HcJ and/or the square appearance of the demagnetisation curve.
Furthermore, taking into account the difficulty of grinding the ferrite, at this stage it is no longer possible to modify the size and/or shape of the elementary particles during this phase of grinding.
The applicant therefore sought a way of making the usual production method more economical and more efficient, while still remaining close, to present technology.
According to the invention, the method for manufacturing M type hexaferrite powders or cakes, of formula AFe12O19, A and Fe being able to be partially substituted, where A refers to a metal chosen among Ba, Sr, Ca, Pb, or their mixture in which:
a) an iron oxide Fe2O3 and an A compound are provided, usually under the form of powders, and a mixture is made of said iron oxide and said A compound, with a molar ratio n=Fe2O3/AO,
b) the said mixture is shaped in the form of agglomerates of shape and size adapted to the calcination stage, and these are calcinated in an oven, usually between 1100xc2x0 C. and 1300xc2x0 C., in such a way as to form the type M ferrite,
c) said calcinated agglomerates are ground to obtain a fine ferrite powder,
is characterised in that
at the previous stage a) of said calcination:
1) said mixture is formed with a ratio n comprised between 5.7 and 6.1,
2) simultaneously with the formation of said mixture or after this, said mixture is ground, so as to have both a mixture with a degree of homogeneity at least equal to a predetermined threshold, and an average particle size of predetermined value comprised between 0.25 and 1 xcexcm,
3) before or during said grinding, one introduces into said mixture an agent for controlling the microstructure (ACM),
at stage b), the calcination conditions together with the nature and content of ACM are chosen in order to obtain, at the end of the calcination phase, a ferrite material under the form of a porous cake also having the following properties:
a transformation yield in crystallised M ferrite greater than 95%,
an apparent density da lower than 3.5 and, preferably, lower than 3, or a porosity higher than 30% and preferably higher than 40%,
low cohesion energy at the grain boundaries between primary particles leading to high brittleness, in order to replace the grinding of stage c) by a simple dispersion of said cake.
The invention thus relates to a combination of essential means.
A first essential means is a choice of ratio n very much higher than the value normally used (5.5). A value n as low as this is used so that a 2nd liquid phase is formed with excess Sr (in the case of a Sr ferrite), this formation of a 2nd liquid phase facilitating the total formation reaction of the ferrite and its crystallisation favouring good anisotropy, which makes the process relatively easy to implement on an industrial scale. But, on the contrary, this 2nd liquid phase has a tendency to weld the primary particles together, making later grinding long and difficult.
The risk associated with a value of n higher than the normal value is an incomplete reaction and poor crystallisation of the ferrite.
The means of the invention thus make it possible to obtain both a ratio n close to stoichiometry (n=6), that is to say to reduce to the minimum the presence of a 2nd liquid phase, while still obtaining a high ferrite transformation yield and a crystallisation sufficient for high anisotropy for the ferrite particles obtained.
A second means is constituted of the formation of a mixture of Fe2O3 and the compound of A with a predetermined and high degree of homogeneity, a mixture associated most often with a third grinding means. In practice, several samples are taken from the mixture, typically 3, generally during the grinding, and these samples are calcinated under standard laboratory conditions.
A mixture is considered satisfactory when one obtains a determined crystallised ferrite yield under these standard conditions. For example, it can be established by tests that a sample leading to a yield of at least 80% in crystallised ferrite with calcination at 1125xc2x0 C. during 30 minutes, will lead to a yield of at least 95% under industrial calcination conditions.
Most often, and especially when one wishes to obtain a ferric oxide of a low particle size (0.6 xcexcm or less), the homogeneity criteria according to the invention is satisfied, during grinding, more rapidly than that relative to the particle size.
A third means is constituted of grinding said mixture so that the average particle size of the primary particles has a predetermined value, comprised of between 0.25 xcexcm and 1 xcexcm.
It is evident that, depending on the grades, more or less fine, of the initial raw materials, the grinding work will be longer or shorter. It can be advantageous to provide sub-micron ferric oxide and to grind it according to the invention to reduce its average particle size by several tenths of a xcexcm and thus obtain the required average particle size.
In particular, it is necessary to obtain a particle size distribution at the end of grinding, which is unimodal and centred on the average size of the primary particles, and not bi-modal with a population of primary particles and another population of aggregates of primary particles of a size typically 5 to 10 times greater than that of the primary particles.
Thus the grinding tends both to reduce the size of the primary particles down to the required level and also to suppress the aggregates of primary particles.
At the end of grinding, the two criteria, that relative to the degree of homogeneity of Fe2O3 and of the compound of A, and that relative to the size of the Fe2O3 particles and of the compound of A, should be fulfilled.
A fourth means is constituted of introducing into said mixture, before or during said grinding, a control agent for checking the microstructure, abbreviated as ACM.
This agent can be incorporated into the crystalline ferrite network during calcination, then precipitated into the grain boundaries, which encourages brittleness according to the observations of the applicant. It can also occur under dispersed form at the surface of the particles or ferrite crystals and thus oppose densification of the ferrite particles during calcination.
Thus, this is also an important means for obtaining a calcinated product under the form of a brittle cake, since it can diminish the cohesion of the particles at the grain boundaries and/or oppose densification during calcination. Tests showed that, in fact, it was possible to act to limit sintering of the primary ferrite particles between themselves during calcination, and thus to diminish the cohesion forces of agglomerates of primary particles.
As will be demonstrated below, the calcination conditions of the ground mixture are not ordinary and they can advantageously be chosen to achieve the aims of the invention.
Only the combination of all these means makes it possible to solve the problem posed and to achieve the aims of the invention, essentially on the one hand a reduction of production and investment costs, especially by reducing the grinding time, and on the other hand the improvement of the magnetic properties through the new possibility of obtaining a more homogeneous ferrite powder with fewer aggregates of primary particles, and finally the possibility of piloting the manufacturing method more closely by intervening on the granulometry of the raw materials just before the calcination stage, and not afterwards as in the classic method.
Furthermore, it should be noted that the significant reduction of time and means for grinding or dispersion after calcination makes it possible to diminish significantly, in the ferrite powders, the Fe2+ content resulting from grinding with iron or steel balls according to the traditional process.